1. Field of the Invention
This invention relates to the field of tensile strength members. More specifically, the invention comprises a method for creating a long tensile strength member with an advanced termination or terminations that can be pre-tested using equipment that is limited to testing shorter tensile strength members.
2. Description of the Related Art
Tensile strength members must generally be connected to other components in order to be useful. A flexible cable provides a good example. The cable must generally include some type of end-fitting so that it can transmit a load. For example, a cable used in a hoist generally includes a lifting hook on its free end. This lifting hook may be rigged to a load. The assembly of an end-fitting and the portion of the cable to which it is attached is generally called a “termination.”
A tough steel lifting hook is commonly attached to a wire rope to create a termination. A “spelter socket” is often used to create such a termination. The “spelter socket” includes an expanding cavity within the end-fitting. A length of the wire rope is slipped into this cavity and the individual wires are splayed apart. A liquid potting compound is then introduced into the expanding cavity with the wires in place. The liquid potting compound transitions to a solid over time and thereby locks the wire rope into the cavity.
The potting compound used in a spelter socket is traditionally molten lead and—more recently—is more likely a high-strength epoxy. However, the term “potting compound” as used in this description means any substance which transitions from, a liquid to a solid over time. Examples include molten lead, thermoplastics, UV-cure or thermoset resins (such as two-part polyesters or epoxies). Other examples include plasters, ceramics, and cements. The term “solid” is by no means limited to an ordered crystalline structure such as found in most metals. In the context of this invention, the term “solid” means a state in which the material does not flow significantly under the influence of gravity.
Terminations on wire rope are quite common in hoists and cranes. These terminations are well understood and their performance and reliability have been established over many decades. In recent years the opportunity to replace wire rope cables with modern, high-strength synthetic cables has arisen. Many different materials are used for the filaments in these synthetic cables. These include KEVLAR, VECTRAN, PBO, DYNEEMA, SPECTRA, TECHNORA, ZYLON, glass fiber, and carbon fiber (among many others). In general the individual filaments have a thickness that is less than that of human hair. The filaments are very strong in tension, but they are not very rigid. They also tend to have low surface friction. They are quite different from steel wires.
Terminations, are used for synthetic filament cables, but they assume a different form than those used for wire rope. Synthetic filament cables tend to be made by braiding multiple strands together. While the individual filaments are made using various modern processes, the construction of the cable itself tends to follow the patterns established for natural-fiber ropes many years ago. Perhaps not surprisingly, the methods used to create a termination tend to follow the old patterns for ropes as well.
FIGS. 1-2 shows a traditional method for adding a termination to one end of a synthetic cable. Cable 10 is made from advanced high-strength synthetic filaments. It is known to join multi-stranded cables using weaving or splicing methods. In these methods, connections are made by interweaving strands of one section of cable with strands of another section of cable (sometimes the sections lie in the same cable and sometimes they do not).
FIG. 1 shows an exemplary prior art operation. Cable 10 includes eight individual strands of synthetic filaments. Each strand may contain thousands or even millions of individual filaments, but the prior art weaving operations do not typically break the cable down beyond the strand level. The depiction of cable 10 is representative rather than entirely accurate. The example shown has 8 separate strands. The strands would typically be interwoven with 2 pairs of strands in a left-hand helix and two pairs in a right-hand helix.
The objective of the example shown in FIGS. 1 and 2 is to weave a length of the cable back on itself to form an “eye” on the cable's end. Considerable mechanical skill and dexterity is required to form an eye on the end of a cable and in other instances to join lengths of cable together. However, persons having these skills are commonly found in industries where large cables are used. Further, the strength and reliability of cable splices made by such persons are well understood and accepted. As a result, it is readily accepted that these proven methods of connection do not require protesting and can even be done in the field, in a non-controlled environment, by trained personnel. This is even true for critical applications. This has been the standard method of termination since inception, and it make up over 99% of the entire industry of long synthetic fiber cables. Thus, there is considerable standardization, knowledge, and field support for such a method of termination.
In FIG. 1, a length of strands proximate the cable's end is unwoven to create separated strands 14. The end of the cable is bent into a loop or bight, sometimes around a reinforcing element such as thimble 12. FIG. 2 shows the continuation of the operation. The weave of the strands within the cable is loosened so that separated strands can be threaded back into the cable in a prescribed pattern. Interwoven section 24 is thereby created. The loose ends of separated strands 14 are typically cut off (after a sufficiently long interwoven section 24 has been created) and taped or otherwise secured.
The result is eye splice 16 on one end of cable 10. When produced by trained personnel, the eye splice does work and it is considered an efficient and reliable means of termination, in this context the term efficiency means the ratio of the breaking stress of the complete cable with the termination attached versus the breaking stress of the cable without a stressed area such as in the middle of the cable. A perfectly efficient cable would have an efficiency of 100%, including the termination. On synthetic fiber cables, achieving this or nearly this efficiency is commonly possible with many forms of prior art splices.
Although the eye splice is strong, it is ill-suited to many applications. For example, while one could use an eye splice to attach a lifting hook to the end of a hoist line, the eye splice is unable to withstand battering forces very well. In addition, the diameter of the eye splice will be too large in many instances. It would be advantageous to instead connect a hook or other device directly to the synthetic cable, analogous to the way a spelter socket is connected to a wire rope. Fortunately, the technology to create such terminations exists.
The prior art approaches to adding a termination to a synthetic cable are explained in detail in commonly-owned U.S. Pat. No. 7,237,336, which is hereby incorporated by reference. The terminations can be added to the cable as a whole or some sub-component of the cable such as a strand. Commonly-owned U.S. Pat. No. 8,371,015 explains how multiple terminations may be attached to multiple strands of a larger cable. This too is incorporated by reference.
In order to gain a strong and repeatable result, the addition of a termination directly to a synthetic cable must generally be done under highly controlled conditions such as found in a factory. This is particularly true of medium to large end fittings configured for a cable having an overall diameter of greater than 20 mm and sometimes being considerably larger. In fact, those skilled in the art recognize that terminating larger synthetic cables is exceptionally difficult to master in even a highly controlled environment. Unlike most metal strength members, achieving an efficient and repeatable result requires very stringent control of the process, highly skilled personnel and precise processing.
An end-fitting is commonly attached to a larger synthetic filament cable by use of a potting compound. Liquid potting compound (such as an epoxy or a polyester) is added to a cavity in the fitting after a length of filaments has been placed within the fitting. It is preferable to hold the components in a stable configuration while the potting compound cures—which may take 12 hours or more. Temperature and other variables are preferably controlled during this process, as are the properties of the potting compound itself. The potting compound may be added to the cavity in a variety of ways, including pre-wetting, infusing, etc.
The process of attaching an end fitting to a synthetic cable produces a wider performance variation than is typical for steel cables or for spliced techniques on synthetic cables. In fact, the creation of an advanced termination on an end of a synthetic cable will often represent the weakest link in the whole system. As such, in many instances it will be necessary to test the strength of the completed termination before it is used.
Exemplary applications include hoisting cables and mooring cables where a known and predictable strength is very important. This requirement creates challenges in the field of synthetic-filament cables since conventional tensile testing equipment used in the industry is (1) limited in length and (2) limited in length. A typical large test frame can pull loads of about 1,000 tons. The length, of such a test frame is only about 20 meters though. Longer test frames do exist (some over 100 meters) but they are very rare. When tensile members are made longer than the length of the readily available test, frame, they are rarely able to be tested properly given the practical constraints that exist in industry. This creates limitations on what can be tested and impacts logistics on any large or remotely used tensile members requiring specialty terminations.
It is desirable to use synthetic filament cables to replace steel and other conventional cables, but in order to do so the synthetic filament cables must have an equivalent useful length. Many large diameter applications are well beyond the typical test bed length, such as 500 or even 1,000 meters as an example. In fact, most large and/or long cables will not fit in any test bed in the world. This complication does not present a serious issue for existing steel or synthetic cables using conventional technology because highly standardized methods and devices have been developed and proven to be reliable over the last century.
By comparison, it is not commonly possible to achieve a the same level of efficiency, reliability, and repeatability with many of the more compact, mechanical, versatile synthetic cable methods of termination such as porting sockets, resin terminations, composite terminations, or spike and cone type frictional arrangements, etc. These types of terminations tend to put far greater stresses in a smaller area, meaning there is much less room for operator error and the efficiency car often be reduced if not handled properly. Further, these types of terminations on synthetic cables have by comparison a limited history, limited use, limited standardization and training, and introduce a significantly greater need for control over the process in order to achieve a repeatable result.
On a synthetic cable, known splicing techniques may not be suitable for long lines and are often not be ideal from a termination perspective. For example, there is often a need for a termination analogous to those used for wire rope. Examples include termination with a hard end such as a hook, a threaded stud, a small eye, or a clevis on the end of a spelter or resin socket, etc. Such hard, versatile, and generally compact ends are well known and thus a desirable option in most industries where large and/or long cables are used. As an example, an offshore steel crane wire would typically have a very compact, potted socket made from steel and including a clevis or eye connection. As with splices used for synthetic fiber cables, these versatile forms of terminations are well established, standardized, trusted, and produced in the field with technicians that are trained in the process. Therefore high load testing in a proofing bed is not generally necessary. However, when considering a high performance synthetic cable, spliced eyes do not provide the same level of functionality or versatility as those from less proven methods.
The proposed method creates a safe and reliable means to apply and validate the performance (pre-test) of a more desirable termination on such long synthetic cable applications. FIG. 3—a sectional view—shows an example of an advanced termination. Anchor 18 includes an internal cavity 20. A length of strands from cable 10 is placed within this cavity. Preferably the strands are splayed apart in some form of expanding cavity (though other techniques may be used). A liquid potting compound is placed within the cavity (either before, during, or after the strands are added).
The liquid potting compound transitions to a solid over time to create potted region 22. Once solidified as shown, the strands within potted region 22 are locked in place and anchor 18 is secured to the end of the cable. Some feature for transmitting a load to the cable is typically included. In this example loading feature 21 assumes the form of a loop.
Other classes of advanced terminations can be made without using a potting compound to secure the cable strands to the anchor. FIG. 10 shows an assembly that is commonly referred to as a “spike and cone” termination. A length of strands is splayed apart in cavity 20 as for the potting example. However, rather than using potting compound, they are mechanically secured. Cone 62 is introduced into the center of the strands. Compression plug 64 is then screwed into the open end of anchor 18 via threaded engagement 66. The strands are then mechanically clamped in place.
It is possible to combine the prior art approaches—such as by using potting compound in the spike-and-cone configuration of FIG. 10. In addition, anchor 18 can be made quite tough. As an example, the anchor may be made of stainless steel so that it can endure an abusive environment. Such a termination, is advantageous in many instances where a synthetic cable might be used to replace wire rope or other more traditional materials.
Countless forms of synthetic fiber cable terminations can be conceived, including those made entirely of composites for example. Any such versatile termination that is not a splice, and especially those which are more compact in nature, have many potential limitations as covered previously. These limitations create the absolute need for production in a controlled setting (which is not in the field).
Given the above, the present industry issue exists: Large ropes are utilized in the field, often in remote areas, and they often need to be re-terminated. Going back to the offshore crane example, if a crane line is damaged in the ocean, there must be an immediate remedy to get hack to work. Removing the line and shipping the cable to its original factory for re-termination is not a feasible option. Reliable field termination of many forms exist for steel wire today, hut they do not exist for synthetic cable. If using a synthetic cable and the termination is anything other than a splice, it requires both a controlled setting and most often proof testing to ensure safe and reliable use. This very fact has prevented synthetic cables from being utilized where a more versatile or compact termination is needed. The present invention presents a solution to this problem, among other problems.